Method of protecting electrolytic cells



P. GALLONE ET AL METHOD OF PROTECTING ELECTROLYTIC CELLS Nov. 5, 1968 2Sheets-Sheet 1 Filed Oct. 4, 1965 INVENTOR% o PATRIZIO GALL NE GIOVANNITRISOGLIO Z w/W/M ATTORNEYS J Hut.

Nov. 5, 1968 P. GALLONE ET AL 3,409,519

ECTROLYT IC CELLS 2 Sheets-Sheet 3 Filed Oct. 4, 1965 INV RIZ O V MN W WATTORNEYS United States Patent 3,409,519 METHOD OF PROTECTINGELECTROLYTIC CELLS Patrizio Gallone and Giovanni Trisoglio, Milan,Italy, as-

signors to Oronzio de Nora Impianti Electtrochimici, S.A.S., Milan,Italy, a corporation of Italy Filed Oct. 4, 1965, Ser. No. 492,526Claims priority, application Italy, Oct. 10, 1964, 52,182/ 64 6 Claims.(Cl. 204-99) The invention relates to a novel apparatus and a novelmethod of protecting the cathode of an electrolytic cell from thecorrosive action of the electrolyte or the electrolysis productsdissolved in the electrolyte as soon as the electrolysis current isinterrupted.

In the following description, for illustrative purposes, reference willbe made particularly to amalgam cells for the electrolysis of alkalimetal chlorides wherein the cathode consists of a flowing layer ofmercury or of an amalgamated metallic surface. It is obvious though thatthe principles of the invention are applicable to other types of cellsthan those specifically described herein.

An electrolytic cell such as a chloro-alkali cell is shut down bycutting oif the electrical current to the cell, but during a period ofinoperation, the mercury cathode of the cell is subjected to chemicalattack by the free chlorine dissolved in the brine remaining in the celland is thus turned into the anode of a short-circuited galvanic batteryand as a consequence, the attack on the mercury in the cell is increasedby the concomitant electrochemical process which takes place within theshort-circuited cell in the presence of the chlorinated brine.

The only practical method of the prior art used to overcome this problemhas consisted of maintaining a cathodic current on the cell to beprotected of suflicient intensity to preserve its immunity. The saidcathodic current is impressed upon the cell by suitable auxiliaryelectrical means according to known methods designated as cathodeprotection methods.

With particular regard to some industrial processes of foremostimportance, the modern trend is to use electrolytic cells of everincreasing size and current capacity, so that eflicient and economicalcathode protection becomes even more diflicult and problematic. As anexample, in a typical mercury cathode cell for alkali chlorideelectrolysis, the results obtained from practical experience indicatethat, in the absence of any protection, the mercury is quickly attackedby free chlorine dissolved in brine. In order to avoid any appreciablebuild-up of mercurous chloride and mercuric chloride when the contactingbrine is saturated with free chlorine at atmospheric pressure, it isnecessary to apply a cathodic current density in the order of 100 A./m.Moreover, cathodic protection must be brought to bear almost at once,and therefore automatically at any interruption of the electrolysiscurrent in order to prevent any appreciable mercury loss and the dangerthat the free chlorine may attack the metal surface of the underlyingcell structure as well. This requires an auxiliary source of cathodiccurrent entirely independent of the power supply for the electrolysisprocess. Therefore, this requirement can only be met by a battery ofaccumulations, particularly since the substitution of the cathodiccurrent for the electrolysis current must be almost instantaneous.

Consider as an example a cell with a mercury cathode surface of 20 m2,which at the rated current density of 6000 A./m. corresponds to acurrent capacity of 120,000 A. Such cell size and current capacity wouldstill be considered economical in a modern chloro-alkali plant notexceeding a daily production of 100 tons/day, but for any larger plantcapacity a larger cell size and current rating would be more profitable.Nonetheless, the auxiliary battery required for cathodic protectionwould have to be rated for a current output of no less than 2000 A., atleast during the residence time of the chlorinated brine inside thecell, which may extend to over 15 minutes before the brine hold up thatwas present in the cell during the electrolysis is evacuated after anyshutdown and replaced with fresh brine. If the latter is thoroughlydechlorinated, the cathodic current density required for cathodicprotection of mercury and steel drops to a very low value of the orderof a fraction of an ampere per square meter. There are, however, someinstances in which the solid salt, used for brine resaturation, is ofsuch a purity grade that only a fraction of the refortified brine streammay be submitted to a chemical treatment and then made to join again theuntreated fraction, so as not to exceed the tolerable level ofimpurities content in the feed stream flowing back tothe electrolysisprocess. In other cases, the available salt is of such a high puritygrade that no chemical treatment is required on the refortified brinestream and, consequently, no dechlorination is needed beforeresaturation. In any such case the cathodic protection battery is to berated at a permanent current output that for the cell size consideredbefore must be no less than 2000 A.

Besides the high cost involved by a storage battery meeting therequirements as outlined above, it must also be remembered that such adevice would in general find application only for applying cathodicprotection to the entire cell bank in case of any planned or accidentalshutdown. It would require an even more expensive arrangement ifcathodic protection were independently provided, which is desirable, toeach single cell during the period when the cell is to be shorted out ofthe electrolysis circuit while the other cells are being kept inoperation. This arrangement requires a considerably more elaborate setup as, for instance, described in US. Patent Nos. 2,834,728 and3,057,984, both granted to Gallone.

Another drawback of cathodic protection resides in that the metalsurfaces receiving the cathodic current tend to become the seat ofhydrogen evolution with the ensuing danger that hydrogen may build up anexplosive mixture not only with the chlorine gas formerly produced byelectrolysis and still present in the system, but also with the chlorinegas and/or oxygen that will develop at the counterelectrode performingas the anode in the cathodic protection circuit as well as with theatmospheric oxygen that might inevitably enter the system.

It is an object of the invention to provide a novel economical method ofprotecting electrolytic cells from corrosive agents during periods ofinoperation.

It is an additional object of the invention to provide a novel method ofprotecting electrolytic cells without cathodic protection.

It is another object of the invention to provide a novel electrolyticcell in which the cell is not subjected to corrosive action duringperiods of inoperation.

These and other objects and advantages of the invention will becomeobvious from the following detailed description.

The novel method of the invention for protecting electrolytic cells fromcorrosive agents during periods of inoperation comprises introducing anon-corrosive fluid onto the surface of the cathode adjacent to theelectrolyte when the electric current to the cell is halted. It ispreferable for the fluid to be introduced to contain a reducing agent toreduce the corrosive agent.

The corrosion rate at which mercury or any other metal, such as iron,are attacked by an aqueous solution of a free halogen, such as chlorine,is strictly related to the rate of diffusion of the free halogen throughthe diffusion layer separating the metallic phase from the bulk ice ofthe solution. It was possible to prove, by experimental investigation,that the diffusion rate is practically a linear function of the partialpressure of the halogen gas. It is, therefore, possible to obtain adrastic fall in the rate of attack and even to reach conditions ofpractical immunity, if the partial pressure of the aggressive agentwithin the bulk of the solution is conveniently diminished, at least inthe immediate proximity of the diffusion layer. This can be obtainedmost conveniently not only by introducing a reactant into the aqueoussolution, so as to chemically reduce the aggressive oxidant, but alsoand much more simply, by bubbling through the solution an inert gas,such as nitrogen or air, onto the immediate proximity of the surface tobe protected.

Whenever the fugacity of the aggressive oxidant such as chlorine, isdepressed by the use of a reducing agent, this may be convenientlydissolved or dispersed in a liquid phase prior to its injection. Typicalexamples of such liquids are aqueous solutions of bisulfite, orsulfurous acid, or thiosulfate. However, if the reducing agent is in thegaseous state, such as sulfur dioxide, it will be even more convenientto have it bubble throughout the bulk of the solution either as such orin admixture with an inert gas by injecting it at points as near aspossible to the surface to be protected.

In order to avoid any contamination of the cell gas delivery line withthe protective gas being bubbled into the cell immediately aftershutdown, it is advisable to intercept the delivery line and establishat the same time a communication between the cell and the vent line assoon as electrolysis is discontinued. This can most conveniently beachieved by means of an automatic control acting on a valve systeminterlocked with the electrolysis circuit breaker or with any otherdevice capable of a response to the ceasing of the electrolytic process.

Referring now to the drawings:

FIG. 1 is a partial sectional view in elevation of a typical mercurycathode cell equipped with graphite anodes.

FIG. 2 is a plan view partially cut out, taken along line II-II of FIG.1.

FIG. 3 is a partial cross-sectional view of the interior of a typicalmercury cell embodying a perforated pipe system for the injection of aprotective fluid onto the mercury cathode when the anodes aredimensionally stable mesh anodes.

In FIGS. 1 and 2 the graphite anode 1 overhangs the mercury cathode 2flowing on cell bottom 3. According to a well known and widely usedarrangement the lower graphite plate surface is provided with slots 4,and the innermost surface of each slot communicates with the top surfaceof the plate through a series of vertical ducts 5, which are drilledthroughout the graphite structure and in combination with the slotsfacilitate the gas release from the anode-to-cathode gap. A suspendingblock 6 of graphite is fixed into a central cavity 20 in the top of theanode plate 1 and is in turn connected with a threaded stem metal 7which passes through the cell cover 8 which, in the present example, ismade of a flexible and plastic material, such as rubber, resistant tothe oxidizing attack of the cell gas, according to the arrangementdescribed in US. Patent No. 2,958,635 granted to De Nora. The openingthrough which the stem 7 is sticking out of the cover is made gas-tightby fastening locknut 9 onto the stem 7, so as to press the cover betweenouter washer 10 and the inner graphite block 6. Another set of two ormore nuts 13 screwed onto the stem 7 serve to establish the mechanicaland electrical connections of the anode with the bus bars and thesupporting structure.

The above-described details, although being already known by themselves,have been illustrated in order to make a clearer distinction with theother novel parts and characteristics of the assembly that are suitableto apply the present invention and will be described hereunder.

The holding stem 7 is bored along its axis and the bore contains a tube12 of chemically resistant material. The stem 7 is screwed into thethreaded cavity 23 worked out in the top of block 6 by means of a wrenchapplied to the hexagonal upper end 15. In this way, the lower end of thestem is pressed against the annular gasket 14 at the bottom of therecess to form a gas-tight assembly and thus prevent any chemical attackof the stem 7 by any anodic gas that might leak through the porosity ofthe graphite block.

The tube 12 ends into a second cavity 18 provided in the bottom of block6 and is held tight by a terminal flange 17 against the innermost cavitysurface, when the tube 12 is pulled from above by screwing nut 15 ontoits upper threaded end until the nut is pressing against the upper endof stem 7.

The lower cavity 18 in the block communicates through radial slots 19with the annular empty space 20 left between the lower recess in theblock 6 and the central top cavity of plate 1, into which the block isfitted. A number of horizontal and intercommunicating ducts 21 are boredthroughout the graphite plate and some of them end in the empty space20. Said horizontal ducts 21 perform the function of a manifold for aseries of vertical d-ucts 22 bored through the bottom of the plate,between the slots 4 and ending in said manifold.

According to the above-described device when a liquid ora gaseous fluidis introduced through the top of tube 12 under a pressure sufficient tooppose the hydrostatic head of the electrolytic solution, it willpenetrate into cavity 18 and through the annular space 20 and ducts 21,22, it will be expelled from the lower anode surface so as to displacethe electrolyte from within the anode-tocathode gap.

In the example of FIG. 3 a mercury cathode cell is schematicallyrepresented as a structure consisting of bottom 30, side walls 31 andflexible cover 32. In this example, the anodes are made of perforatedmetal structures 33 suspended from metallic stems 34. Such anodes have arelatively small thickness and have a large ratio of empty to solidspace so that any fluid introduced into the electrolyte just above theanodes will easily pass through the perforated structure and reach theinterelectrodic gap. Accordingly, the method of the invention can beeffected by means of a relatively simple arrangement to take advantageof the dimensionally stable anodes which are extremely stable due to thevery limited amount of chemical and mechanical wear, so that they can begiven a permanent adjustment without the structural arrangements thatwould be needed to adjust for gradual wear in the course of operatinglife. It is, therefore, simple to arrange immediately above these anodesa set of horizontal perforated pipes 35 through which the protectivefluid. will be injected into the cell. The perforations through eachpipe will preferably be limited to that part of its surface that isfacing the cathode.

Whenever the active surface of the perforated anodes 33 consists of athin coat of a noble metal such as platinum, iridium, rhodium or ofnoble metal alloy, the protective method of the invention acquiressignificance also as regards the anodes if applied in a cell with amercury cathode. Indeed one of the advantages afforded by thedimensionally stable structure of the latter resides in the possibilityto give the anodes a permanent adjustment in such a way as to leave onlya very narrow distance from the mercury cathode surface with anintermediate gat of no more than 3 mm. The voltage losses caused by theelectrolyte resistance are thus substantially reduced in comparison withthe performance that it is possible to achieve with graphit anodees.With graphite anodes, it is well known that any decrease of theinterelectrodic distance to less than 5 mm. would bring about hardly anyimprovement and on the contrary, because of the difficulty for the gasto escape away from the gap, the results would thereby become worse inmost cases, whatever the slot and hole configuration of the graphiteplate.

However, the drawback of such a narrow gap generally resides in thedanger of the anodes becoming contaminated with solid calomel in case ofa cell shutdown if the aggressive action of the dissolved free chlorineor mercury is not promptly suppressed. In fact, such contaminationbrings about a scaling on the thin coat of noble metal and by bridgingthe gap between the anode and cathode surface may cause a local shortcircuit. Consequently, on restarting the electrolysis current, the thincoat of noble metal may become seriously deteriorated. However, bythoroughly suppressing any calomel formation by the present invention,not only will the mercury inventory remain unimpaired, but it will alsobe possible to make use of all the advantages afforded by these anodessince the narrower distance at which they can be adjusted from thecathode will allow operation at a lower voltage without any danger fortheir integrity.

Various modifications of the method and apparatus of the invention maybe made without departing from the spirit or scope thereof and it is tobe understood that the invention is to be limited only as defined in theappended claims.

We claim:

1. A method of protecting electrolytic cells from corrosive agentsduring periods of inoperation which comprises introducing anon-corrosive fluid onto the cathode surface adjacent to the electrolytewhen the electric current to the cell is halted.

2. The method of claim 1 wherein the fluid is a chemically inert liquid.

3. The method of claim 1 wherein the fluid is a liquid containing anagent for reducing the corrosive agent.

4. The method of claim 1 wherein the fluid is a chemically inert gas.

5. The method of claim 1 wherein the fluid is in the gaseous state andcontains an agent for reducing the corrosive agent.

6. A method of protecting an alkali metal electrolysis cell having aflowing mercury cathode from the corrosive effects of chlorine duringperiods of inoperation which comprises introducing a non-corrosive fluidonto the mercury cathode surface adjacent to the electrolyte when theelectric current is halted.

References Cited UNITED STATES PATENTS 1,565,943 12/1925 Klopstock204-99 2,834,728 5/1958 Gallone 204147 3,310,482 3/1967 Bon et a1.2042l9 FOREIGN PATENTS 316,694 8/ 1929 Great Britain. 988,610 4/1965Great Britain.

HOWARD S. WILLIAMS, Primary Examiner. D. R. JORDAN, Assistant Examiner.

6. A METHOD OF PROTECTING AN ALKALI METAL ELECTROLYSIS CELL HAVING AFLOWING MERCURY CATHODE FROM THE CORROSIVE EFFECTS OF CHLORINE DURINGPERIODS OF INOPERATION WHICH COMPRISES INTRODUCING A NON-CORROSIVE FLUIDONTO THE MERCURY CATHODE SURFACE ADJACENT TO THE ELECTROLYTE WHEN THEELECTRIC CURRENT IS HALTED.